This invention relates to roller hearth kilns and in particular to mechanisms for supporting and driving rollers in the kilns.
A roller hearth kiln is used in both the ceramic and metallurgical industries and includes an elongated, tunnel-like insulated process chamber which has means for providing input of heat and movement of a process load therethrough. Typically the center third of the tunnel length but often as much as 70% of the length is heated. Product to be treated or fired is transported through the length of the process chamber on a conveyer system consisting of closely-spaced rollers, placed at 90.degree. to the direction of travel. Rollers are normally made of high-temperature resistant materials, including various metal alloys, and a variety of high-strength ceramic materials. Ceramic rollers are the most common in the ceramic process industry.
A normal practice within the industry is to use rollers that are sufficiently long to extend through insulated sidewalls of the kiln, such that support of the rollers and the application of torque (driving force) can be accomplished outside the kiln and at the lower temperatures found there. Every roller kiln requires a drive system that performs the following functions:
1. Support of the rollers at both ends of each roller.
2. Retaining of the rollers within the system.
3. Reduction of turning friction of the rollers.
4. Imparting of torque to the rollers so that they may in turn convey the load.
5. Adjustment of roller level and perpendicularity relative to the direction of travel so as to provide tracking adjustment.
Within the industry a prime concern of customers relative to the roller hearth kiln is the function and serviceability of the roller and drive system as it is often the most difficult part of the process system to implement and normally requires more long-term maintenance and investment than all other components combined.
Two factors that affect those long-term costs are:
1. The rollers require periodic replacement due to breakage.
2. The drive system involves thousands of moving and wearing parts.
Thus one object of the invention includes effecting a smooth stress-free transfer of power to the rollers as such can greatly reduce roller replacement rates and even roll replacement costs due to drive shock and support system induced stress.
A further object of the system is to provide power distribution which is smooth so as to reduce mechanical issues and costs.
Current roller hearth kiln drives typically fall into two classes. The common system is to drive ceramic rollers using a coupling that employs a drive shaft that supports a drive cup for retaining a roller end and a steel cross pin or metal shaft that connects the cup and the roller. The drive shaft is supported by one or two bearings outside the kiln side walls. A sprocket is mounted on a shaft between the bearings and a chain of matching pitch is drawn over or under the sprockets to turn them. Various arrangements are used resulting in drive loops as short as five feet or as long as 100 feet. Problems inherent in this system include:
1. Chain stretch;
2. Stick slip behavior when the system is heavily loaded;
3. Poor tolerance for overrunning loads; and 4. High stress on bearings due to chain tensions required to overcome problems associated with numbers 2 and 3 above.
It is thus another object of this invention to minimize or eliminate the problems associated with a drive chain.
A commonly used alternative for roller driving involves a series of gears mounted on the roller shafts with counter or idler gears mounted therebetween so as to engage adjacent shaft gears and couple power from one roller to the next. As many as twenty shafts may need to be driven in a given section, resulting in forty gears and bearing sets in line and the first gear transmitting all the power required for every roll in the section. Thus a single broken tooth on any one of the first gears can result in the whole section stopping. Thus the load rating of the section must be carefully limited.
A further object of this invention is to minimize the problems which have occurred because of such a gear system.
Once the drive torque has been coupled to the bearing-mounted shaft, it still remains to effectively and safely couple that to the roller itself and to support the roller in a uniform and adjustable manner. A common usage is a system where the drive shaft has an integral cup and the cup is slightly larger than roller outside diameter and is provided with a transverse pin that engages a slot or notch machined into the roller. Support and retention of the roller is accomplished by a corresponding idler cup on the non-driven end of the roller which is mounted in a device similar to the drive assembly but without the drive sprocket or gear. A spring and collar or keeper arrangement keeps the idler cup firmly engaged to the roller and therefore the roller firmly engaging the drive cup. However, this system also has certain disadvantages. For example:
1. The cup by being slightly oversized from the roller may cause tumbling of the ceramic roller within the cup. Under load this may eventually result in breakage of the ceramic roller.
2. Removal of the roller requires complete disassembly of the idler cup, spring and bearing assemblies.
3. Since the cup is rigidly fixed to a shaft, there is little tolerance within the system for roll warpage or pressure, often resulting in shortened roller life. This problem then mandates low precision bearings within the support and drive system. Thus to run properly the system must be "sloppy".
Yet a further object of this invention is to provide a coupling system which avoids the disadvantages stated hereinbefore.
An alternative coupling and support system is used to provide a drive cup with some clamping means to affix it to the roller. The opposite end of the roller is then rested or supported on the opposite side in a notch or nip and between closely-spaced idler wheels or bearings. This system addresses problems listed hereinbefore but fails to address roller warpage, camber or loaded deflection.
Another factor which is believed to be critical is the fact that these kilns run at very high temperatures and thus the drive systems although outside the kiln are exposed to significant ambient temperatures.
It is thus desirable to maximize reliability of the drive bearing systems in such an application.
These and other objects of this invention will become apparent from the following disclosure and appended claims.